Synchronizing mechanism



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SYNCHRONI Z ING MECHANI SM Filed gan. 19,v 1944 s Sheets-Sheet 1 KVH 12 13 14 15 RADO PICTURE PCTURE FREQUENCY DETECTOR SIGNAL AMP HER cmcun L i SYNCHRONOUS svucuaonovs SIGNAL SIGNAL 36 AMPLIFIER FILTER sw 51 W;

102 L 41 w? J 7 July 23, 1946. -.w. G. H. FINCH 2,404,571

SYNGHRONIZING MECHANISM Filed Jan. 19, 1944 3 sheets-sn ei 2 76 74 SYNCHRONOUS 8 -v- D.C

MOTOR MOTOR 84 441 ATTORNEY g H g cg-g SYNGHRONIZING MECHANISM Filed Jan. 19, 1944 3 Sheets-Sheet 5 INVENTOR m/ ATTORNEY Patented July 23, 194-6 UNITED STATES PATENT OFFICE SYNCHRONIZING MECHANISM William G. H. Finch, Newtown, Conn.

Application January 19, 1944, Serial No. 518,837

8 Claims. 1

My invention relates in general to the field of picture transmission and reception and more specifically to a novel continuous synchronizing mechanism,

In facsimile transmission, a signal is generated at the transmitter which represents the light shades of a single line of a picture. This signal is then amplified to a suitable energy level and transmitted by wire or radio to a remote receiver.

As is well known in the art of facsimile, it is essential that both transmitting and receiving apparatus operate in synchronism. In the past, synchronizing systems have primarily been stopstart mechanical systems wherein the receiver recording mechanism was operated at a somewhat higher speed than the transmitting mechanism. The scanning of a single line of the picture at the receiver was thus accomplished in less time than a corresponding cycle at the transmitter, and the recording mechanism was brought to a halt to await the receipt of a synchronizing signal from the transmitter. This synchronizing signal representing the start of a new scanning cycle at the transmitter would release the receiving mechanism to record the following line. The mechanical disadvantages of such a start-stop mechanism are well known in the facsimile art.

It is possible to obtain continuous synchronizing of transmitting and recording mechanism if both are operated from the same alternating current power supply. In thi ideal instance, matched alternating current synchronous motors are utilized to provide absolute continuous synchronization between the rotational elements at each station. Obviously, such a transmission system would be limited to the area served by the one central power station and would be of no use whatsoever for facsimile systems operating between separated points such as ship-toshore or airplane-to-ground, or the like.

Continuous synchronization of transmitting and recording equipment may also be obtained by the continuous generation at the transmitter of an alternating current synchronizing signal which is transmitted to the receiving station and amplified thereat to a level surficient to drive all the required recording apparatus. With modern high speed telepicture apparatus, this particular synchronizing system becomes impractical, since the power requirements of the recording mechanism are comparatively high and require excessive amplification of the synchronizing signal by massive, ineflicient electronic apparatus.

My present invention proposes continuous synchronization between transmitting and recording apparatus which utilizes a continuously transmitted alternating current synchronizing signal, but which does not require the excessive amplification thereof mentioned above. In accordance with my novel synchronizing mechanism, the recording apparatus is driven from any local power supply, which may take the form of alternating current mains, or a portable battery as employed in an airplane or other vehicle.

If a battery supply is available at the receiver then the drive mechanism may consist of an ordinary direct current shunt motor of the required horse power. The speed of this shunt motor is controlled directly by comparatively simple and light apparatus which in turn is governed by a small pilot synchronous motor operated from a low level continuous synchronizing ignal amplifier. Thus the transmitting apparatus has associated with it means for generating a continuous synchronizing signal and combining it with the picture signal to be transmitted. This multiple signal when received at the recording apparatus is separated electronically and the picture signal is passed through a recording channel while the synchronizing signal is impressed upon an amplifier having a signal output sufficient to drive the aforementioned small synchronous motor.

Immediately upon the receipt of a transmitted signal, the pilot synchronous motor operates to adjust the speed of the direct current motor to that of the corresponding scanning apparatus at the transmitter. Any fluctuation in speed of the direct current motor will thereafter be immediately compensated for by the small pilot motor and result in a change of electrical or mechanical circuit constants that will restore the speed of the direct current motor to the required synchronous speed.

It is therefore an object of my invention to provide a facsimile transmitting and receiving system wherein the recorder is continuously governed to operate at the speed of the transmitter.

It is another object of my invention to provide a continuous telepicture synchronizing system utilizing a small pilot synchronous motor at the receiver.

Still another object of my present invention is to provide facsimile recording apparatus which may be operated from any power source whatsoever and which is maintained in synchronism by a continuously transmitted governing signa1 generated at the transmitter.

A further object of my invention is to provide 75 a novel, differential speed adjusting means for maintaining a direct current motor in synchronism with a synchronous alternating current motor.

A still further object of my invention is to provide a novel electrical circuit for controlling the speed of a direct current motor within certain narrow pre-determined limits.

These and other object of my invention will now become apparent from the following specification taken in connection with the accompanying drawings in which:

Figure 1 is a schematic representation of one modification of my novel continuou synchronizing mechanism.

Figure 2 is a fragmentary View of the diiferential mechanism for governing the speed of a direct current motor.

Figure 3 is a fragmentary view of one form of speed control means; and

Figure 4 is another modification of a motor speed control mechanism.

Referring now to Figure 1, there is shown a schematic representation of a facsimile system operable from radio signals received by antenna ii and coupled to the radio frequency receiving circuit E2.

The facsimile signal received in circuit i2 consists of a carrier signal modulated by audio-frequency currents representing the variations of light along individual lines of an image. These signals are generated at the transmitter as is well known in the art by an electro-optical scanning mechanism driven from a suitable source of power.

If an alternating source of power supply is available at the transmitter, then the transmitting scanning mechanism is driven by an alternating current synchronous motor, To obtain a signal at the transmittenwhich is representative of he speed of the synchronous motor, it is then merely necessary to modulate the carrier with this alternating current,

If it is necessary to drive the transmittingscanning mechanism from a battery or other D. C. supply, it is then preferable to attach a small synchronous alternator to the scanning drive motor so that an alternating current signal having a frequency proportional to the speed of the canning apparatus is generated at all times.

As in the previous example, the output of this synchronous alternator is used to modulate the carrier to provide a transmitted synchronizing signal.

When wired transmission is employed, the picture signal and the continuous synchronizing signal may be directly app-lied to the lines and modulation of a carrier and the receiving radio frequency circuits may be dispensed with.

Since the picture currents and the synchronizing signal may be in the same frequency band, multiple transmission may be accomplished by translating either one of these two into another frequency band by the addition of a constant fre quency signal thereto. At the receiver, as will be described, the two signals are separated and the translated current restored to its original frequency.

Again referring to Figure 1, the received signal is demodulated by the detector l3 and impressed upon the two-channel electronic circuit comprising the picture signal filter l4 and the synchronizing signal filter H5. If the synchronizing signal is, for example, an alternating current signal of approximately 60 cycles per second and the picture signals a translated band of higher frequencies, the filters i l and it may be conventional high-pass and low-pass filters respectively.

The audio-frequency picture signals are amplified by amplifier l5 and impressed in the well known manner across an electrosensitive recording sheet H, which may be a sheet of instantaneous facsimile recording paper of the conductive type.

As is schematically illustrated in Figure 1, the recording sheet El is mounted upon a rotating drum 2i by a mounting bar The drum 2! is supported by suitable bearings 23 and is driven by a motor 24 of the required horse power. Scanning the surface of the recording sheet I! is an electrically conductive stylus 25, supported on a suitable bracket 23, which in turn is driven by a conventional feed screw mechanism.

The picture signal output of amplifier i5 is impressed between the stylus 25 and the rotatable drum 25 whereby the incoming picture signal will fiow through the recording sheet ll and in the well known manner produce an instantaneous recording.

It will become evident that the present invention is not particularly limited to the recording drum 2! as illustrated in Figure 1, and that any other well known facsimile recording mechanism, such as the multiple stylus arrangement, or the reciprocating arm arrangement, may be directly substituted for the particular recorder described.

The power source comprises a battery or other direct current supply 3!. The source of motive power for the drum 2i, as shown, is a direct current shunt motor 2 5, the armature terminals 32 of which are directly connected to the battery through a switch 33. The field of the direct cur rent shunt motor 2 5 is also energized from the battery 5! through the field rheostat 34 and through a circuit contained in the speed control apparatus 35, to be described later.

The alternating current synchronizing s gnal flowing from the signal filter i6 is impressed upon a synchronizing signal amplifier 36 having a power output sufiicient to drive the small pilot synchronous motor 3? It is important to note this point that the direct shunt motor 24 is considerably larger than the pilot synchronous motor 3? and that the energy required to drive the recording drum 25 is supplied primarily by the direct current motor 2 1 operating from the local direct current source 35.

The output of the synchronizing signal amplifier is an alternating current synchronous with the alternating current utilized to drive the transmitting scanning mechanism, and is impressed directly upon the armature terminals M of the pilot synchronous motor 31.

The direct current field terminals 42 of the pilot synchronous motor 37 are energized from the local power supply 3! through the four-pole relay contacts The relay M is picture signal operate-d and of the latching type. That is, upon the receipt of a picture signal through picture amplifier i5 the coil 45 of the relay M will be energized across the lines 48. The relay contact carrying arm i? is normally biased as indicated sche matically by spring 53 to the open circuit position illustrated.

Upon receipt of a picture signal over lines 46, the relay coil 55 will be energized and cause the movement of contact carrying arm 47 to the right as viewed in Figure 1, and accordingly close the four circuits illustrated.

Eince relay at is of the latching type, any subsequent temporary cessation of the picture signal,

will not cause the contacts 43 to open. The closure of contacts 52 and 53 of the relay 44 will, as illustrated, energize the direct current field at terminals 42 from the local D. 0, supply 3|, and in combination with the alternating current signal applied to the armature terminal 41, will cause the synchronous motor 3'1 to look into synchronous speed, as is well understood in the art. The speed of this synchronous motor 3i is accordingly the speed at which it is desired to operate the direct current shunt motor 24. And as will be described later, the differential speed control mechanism 35 will in connection with the synchronous motor 3'1 provide means for continuously adjusting the speed of the shunt motor to maintain the desired scanning speed.

The mechanism is a mechanical coupling between the pilot synchronous motor 31 and the direct current shunt motor 24. Since it may be desirable to operate the shunt motor 24 when no picture signal is being transmitted, it is necessary to uncouple the mechanical linkage between the motor 24 and the synchronous motor 3'5. This is accomplished by a clutch mechanism operated by arm Bl pivoted at pin e2.

The normal position of the pivoted link ill is determined by the spring-53 applying tension in the direction illustrated in Figure 1. However, upon the energization of relay 45 and the closure of contacts 64 and 65 by the contact carrying arm 41 the solenoid 86 will be energized and cause counterclockwise rotation of pivoted link 6i against the normal bias of sprin 63. This, as

will be illustrated later, will cause the mechanical l coupling between the synchronous motor 3'! and shunt motor 24 to maintain the desired synchronous condition.

Referring now to Figure 2, there is illustrated one possible mechanical structure of the synchronizing linkage between the pilot motor 31 and the shunt motor 24.

As illustrated, the synchronous motor 3? drives shaft H to which is secured bevel gear 72. I he attachment is accomplished, as illustrated by pin 13 passing through the hub of gear 12 and through shaft 1 l.

The direct current shunt motor 24 drives shaft 14 to which bevel gear i5 is slidably but nonrotatably keyed. Thus, keyways milled in shaft 74 and bevel gear 15 engage a key 16 which is secured to the shaft 14. The keyway in bevel gear 15 is somewhat larger than the key 16 and hence the bevel gear 15 may move to the right or to the left as viewed in Figure 2.

The movement of bevel gear 15, is determined by the movement of coupling link 6i illustrated in Figures 1 and 2. The link 6| slidably engages the hub of the bevel gear 15 in a cylindrical depression T! wherea't friction has been reduced to a minimum by proper polishing.

The position of bevel gear '15 as illustrated in Figure 2, corresponds to the energization of solenoid 66 shown in Figure 1. Upon de-energization of this solenoid 6B, the spring 63 will act to move bevel gear 75 to the right, as viewed in Figure 2, and cause the disengagement of the gear (5 with another bevel gear 81. The bevel gear BI is rotatably positioned upon a shaft 82 by bushings 83 and 84 pinned to the shaft by pins 85 and 86.

The shaft 82 is rotatably positioned by fixed bushings SI and 92 upon a link 93 which is an integral extension of a large bevel gear 94. The bevel gear 94 is rotatably positioned upon the synchronous motor shaft H between the hub of bevel gear of 12 and a inned bushing 95.

t will now become evident that with the linkv speed of the synchronous motor driving shaft ll,

it is evident that bevel gear M will continue to rotate about shaft 82 but will, in addition, exert a force upon the fixed link 93 which will tend to drive the bevel gear 94 about shaft 1 I.

The angular speed of bevel gear 94 is directly dependent upon the different speed between the D. C. motor and the synchronous motor, and the direction of rotation thereof is dependent upon whether the speed of the D. C. motor is greater or less than that of the synchronous motor.

correspondingly, the rotation of shaft 91 and the direction thereof is directly a measure of the non-synchronous condition of the D. C. motor and the direction of this deviation. As is evident from Figure 2, the angular movement of bevel gear fit will be considerably greater than that of engaged bevel gear 234 and hence, is an extremely sensitive detector of this aforementioned nonsynchronous condition. y

In accordance with my invention, I propose to utilize the rotation of shaft 9? to maintain a facsimile recorder at the speed of a corresponding unit at the transmitter.

The speed of the shunt motor 24, as is well known in the art, is determined by the magnitude of its shunt field current which in turn is determined by the variable resistor 34, and additional electrical means in the differential mechanism 35. The means of accomplishing the speed control of direct current shunt motor 26 is illustrated in Figure 3.

The variable resistor I0! is connected in series with the shunt field circuit at the terminals we, as illustrated, and hence the complete shunt field circuit comprises resistors 34 and lill, and the field itself.

The bevel gear 96 and the attached shaft 91, which is preferably of an insulating material, control the magnitude of the resistance ml inserted into the shunt field circuit. Thus, conductive arm M23 is rigidly attached to shaft 91 by means of a pin 34, and completes the circuit from the rheostat it! through the lines Hi2.

When the D. C. motor begins to run below the synchronous speed, arm I23 will tend to rotate in the direction of the arrow S, as illustrated in Figure 3. Such rotation will increase the resistance in the shunt field circuit, thereby decreasing the current and causing an immediate increase in the armature speed thereof.

It is important to note that it is desirable to have the resistor it! of such magnitude that extremely small rotation of the arm Hi3 will cause a considerable variation of resistance in the shunt field circuit, thus resulting in an immediate tendency to restore the speed. Conversely, any tendency of the direct current motor to run at above the synchronous speed of the motor 31 will cause the arm N13 to rotate in the direction F, as illustrated in Figure 3, and result in an increase. of

field current which will tend to restore the speed to that which corresponds to no rotation of the bevel gear 96.

It is therefore evident that by utilization of an extremely small synchronous motor governed by signals transmitted from the scanning apparatus at the transmitting station, I have been able to synchronize a comparatively large direct current motor thereby and insure proper operation of the recording apparatus 2|.

The power requirement of the synchronous motor 37 is extremely small, and as may be seen from Figure 2, is only that required to overcome friction of the gears, and the rheostat mechanism. The synchronous motor 37 does not to any extent contribute directly to the driving force required by the rotatable drum and scanning mechanism 2|.

Another modification of my novel speed control mechanism is illustrated in Figure 4, and utilizes the differential gear mechanism illustrated in Figure 2. Thus, difierences in speed between the D. C. motor and the synchronous motor results in rotation of bevel gear Q6 and attached shaft 91, the direction being dependent upon the relative speeds thereof.

As illustrated in Figure 4, a link III of insulating material is attached at one end to shaft 97 by means of a pin H2, and carries at the opposite end thereof a conducting contact bridge H3. The length of contact bridge H3 is sufficient to interconnect contacts I I4 and I I 5 which are fixed to the frame of the facsimile apparatus. As illus trated in Figure 4, the circuit between contacts I I4 and I I5 is not completed by the bridge I I3.

For this modification, a fixed resistor lid is connected in series with the variable resistor 34 and the shunt field of motor 24 at the terminals I62, as is illustrated in Figure 1. Contacts Ill and H8 disposed at the ends of resistor IIfi are engageable by the armature I2I of a relay I22. The armature I2I and an additional contactor I23 of the relay I22 are normally biased by the spring I24 and I25 respectively, into position wherein they are normally disengaged from the contacts III, II8 and I26.

Energization of the relay I22 will cause the displacement of armatures IZI and I23 against the normal spring bias illustrated, and cause the engagement of the armatures with their respective contacts. With the contact bridge I I3 in the position illustrated in Figure 4, the resistor III} is part of the shunt field circuit. This resistor I I6 has been adjusted in connection with the variable resistor 34 so as to provide a speed of the I D. C. motor 24 which is above that of the syn chronous motor 3i when in circuit and a speed lower than that of motor 3? when shorted out of the circuit.

As previously described in connection with Figure 2, such a condition of relative speed will cause the rotation of bevel gear 96 and hence, of link I I I in the direction marked F in Figure 4.

The continued rotation of the link III and its associated contact bridge I I3 will ultimately cause the closure of contacts I I4 and I I5. Closure of the contacts II4 and H5 will, through wires I3I and I32 and the battery supply I33, cause the energization of the coil I34 of relay I22 which in turn will cause the displacement of armatures I2I and I23 against their normal spring bias.

The engagement of contacts I I I and M8 by the armature I 2] will short-circuit the resistor IIS and hence reduce the total resistance in the shunt field circuit of the motor 24. This reduction will immediately result in an increase in field current which correspondingly causes a decrease in the speed of the motor 24. This decrease in speed will through the diiferential mechanism illustrated in Figure 2, immediately re-act to cause the rotation of bevel gear 96 in the direction marked S in Figure 4. Continued rotation in this direction will first cause contacts II 4 to disengage from the contact bridge II3. However, this disengagement of contact II4 which accordingly results in an open circuit between contacts H4 and H5 will not cause the de-energization of contact relay coil I34, since this relay, when first energized, caused the engagement of armature I23 with contact I25, which through wires I35 and I 36 serves to supply an alternate path for energization of the coil I34.

Accordingly, the short-circuiting of resistor I I 6 which tends to slow down the shunt motor 24 will continue even though I I4 has been disengaged by the arm H3.

As the shunt motor continues to run below the speed of the synchronous motor, the arm I I I will continue to rotate in the direction F of Figure 4 until the contact bridge II3 leaves the contact IIE, under which circumstances both paths through which relay coil I34 could be energized are open. This will immediately cause springs I24 and I25 to open circuit contacts H7, H8 and I26 resulting in the re-insertion of resistor II6 into the field current circuit of the shunt motor 24.

Accordingly, as previously mentioned, this will again, due to the predetermined value of resistor I I 6, result in an increase of speed in the motor 24.

Consequently, it may be seen that the electromechanical circuit illustrated in Figure 4, will serve to maintain the shunt motor 24 in a nonsynchronous condition by maintaining th speed either above or below the synchronous speed of motor 3?. However, it is important to note that the angle of rotation of arm I II is comparatively great for even extremely small differences in speed between the pilot motor 31 and the shunt motor 24. Evidently, therefore, the departure from synchronous speed can be made to be extremely small by the continuous and rapid opening and closing of the contact arm I2I of the relay I22. That is to say, that the shunt motor 24 when operated by the control mechanism illustrated in Figure 4, will be continuously hunting, that is, fluctuating about the synchronous speed of the motor 37, and Will on the average run at synchronous speed.

The maximum departure from synchronous speed as determined by the angular displacement of contacts H4, and H5, will be so small as to make the effect upon a straight line in a picture transmitted indiscernable.

Summarizing, therefore, the circuits of Figure 3 and Figure 4 when utilized with the mechanical linkage between the synchronous motor and the shunt motor illustrated in Figure 2, comprise a overnor of the speed of motor 24. However, it is evident that the governor thus formed is at all times responsive to changes in synchronous speed which may be encountered at the transmitter, particularly if the transmitter as previously described is operated by a battery which in turn serves to generate an alternating current signal for synchronizing signal transmission.

If, as previously mentioned, between picture transmission it is desirable to operate the direct current motor 24, the latching relay 44 may be released to the open circuit position illustrated in Figure 1. This will immediately de-energlze the solenoid 66 and cause the rotation of link 6i under the influence of spring 63 as illustrated in Figures 1 and 2. This will cause slidably positioned bevel gear 15 to move to the right and result in disengagement thereof from bevel gear 8|.

Accordingly, the mechanical linkage through the synchronizing mechanism as illustrated in Figure 2, will not be complete between the shafts II and 74.

The synchronous control mechanism described in the above paragraph obviously may be extended to the speed control of various other types of motors. Thus, the shaft 91 controlled by rotation of the bevel gear 96 may be used to actuate mechanism to control the speed of various other forms of motive power including the Well known alternating current speed control motors.

Accordingly, I do not wish the scope of the present invention to be limited to the particular modifications described above, but to be bound by the appended claims.

I claim:

1. In a picture transmission system, a transmeans for impressing said alternating current signal upon a synchronous pilot motor, said driving motor and said synchronous motor being mechanically coupled, said coupling comprising comprising means operative upon a speed difference between said motors to adjust the speed of said drive motor to the speed of said synchronous motor.

2. In a picture transmission system, a transmitter for generating signals in accordance with the lights and shades of a picture, means for generating a continuous alternating current signal of a frequency representative of the speed of said transmitter, means for transmitting said signals, a receiver for receiving said signals, means for impressing said picture signals upon a picture recording mechanism, means for driving said recording mechanism, comprising a motor, means for impressing said alternating current signal upon a synchronous pilot motor, said driving motor and said synchronous motor being mechanically coupled, through a differential mechanism having an element responsive to the difierence in speed between said motors, said element being continuously operative to maintain the speed of said driving motor at the speed of said synchronous motor.

3. In a picture transmission system, a transmitter for generating signals in accordance with the lights and shades of a picture, means for generating a continuous alternating current signal of a frequency representative of the speed of said transmitter, means for transmitting said signals, a receiver for receiving said signals, means for impressing said picture signals upon a picture recording mechanism, means for driving said recording mechanism comprising a direct current motor, means for impressing said alternating current signal upon a synchronous pilot motor whereby said synchronous motor is continuously operative at the speed of said trans- 10 mitter, a difierential coupling gear between said direct current motor and said synchronous motor having an element responsive to speed differences between said motors, said speed responsive element being continuously operative to adjust the field of said direct current motor to maintain said direct current motor at tho speed'of said synchronous motor.

4. In a picture transmission system, a transmitter for generating signals in accordance with the lights and shades of a picture, means for generating a continuous alternating current signal of a frequency representative of the speed of said transmitter, means for transmitting said signals, a receiver for receiving said signals, means for impressing said picture signals upon a picture recording mechanism, means for driving said recording mechanism, comprising a motor, means for impressing said alternating current signal upon a synchronous pilot motor, said driving motor and said synchronous motor being mechanically coupled, said coupling comprising means to cause said recording motor to fluctuate in speed, said fluctuations being rapid and continuous and within narrow predetermined limits above and below the speed of said synchronous motor.

5. In a picture transmission system, a transmitter for generating signals in accordance with the lights and shades of a picture, means for generating a continuous alternating current signal of a frequency representative of the speed of said transmitter, means for transmitting said signals, a receiver for receiving said signals, means for impressing said picture signals upon a picture recording mechanism, means for driving said recording mechanism, comprising a motor, means for impressing said alternating current signal upon a synchronous pilot motor, said driving motor and said synchronous motor being mechanically coupled, said coupling comprising means to cause the speed of said recording motor to first rise above the speed of said synchronous motor and operative when said recording motor has reached a predetermined upper speed limit to cause said speed to fall below the speed of said synchronous motor, whereby the average speed is equal to the speed of the synchronous motor.

6. In a picture transmission system, a transmitter for generating signals in accordance with the lights and shades of a picture, means for generating a continuous alternating current signal of a frequency representative of the speed of said transmitter, means for transmitting said signals, a receiver for receiving said signals, means for impressing said picture signals upon a picture recording mechanism, means for dI'iVlllg said recording mechanism comprising a direct current motor, means for impressing said alternating current signal upon a synchronous pilot motor whereby said synchronous motor is continuously operative at the speed of said transmitter, a, differential coupling gear between said direct current motor and said synchronous motor having an element responsive to speed differences between said motors, said element being operative continuously to insert or remove resistance from the field circuit of said direct current motor whereby the speed of said direct current motor is caused to rise above and fall below the speed of said synchronous motor.

'7. In a picture transmission system, a transmitter for generating signals in accordance with the lights and shades of a picture, means for generating a continuous alternating current signal of a frequency representative of the speed of said transmitter, means for transmitting said signals, a receiver for receiving said signals, means for impressing said picture signals upon a picture recording mechanism, means for driving said recording mechanism comprising a direct current motor, means for impressing said alternating current signal upon a synchronous pilot motor whereby said synchronous motor is continuously operative at the speed of said transmitter, a differential coupling gear between said direct current motor and said synchronous motor having an element responsive to speed differences between said motors, said element cooperating with an electrical time delay circuit to insert resistance into the field circuit of said direct current motor until the speed of said direct current motor rises to a predetermined speed above the speed of said synchronous motor and then to remove resistance from the field circuit of said recording motor until the speed of said recording motor falls to a predetermined speed below the speed of said synchronous motor.

8. In a picture transmission system, a transmitter for generating signals in accordance with the lights and shades of a picture, means for generating a continuous alternating current signal 12 of a frequency representative of the speed of said transmitter, means for transmitting said signals, a receiver for receiving said signals, means for impressing said picture signals upon a picture recording mechanism, means for driving said recordin mechanism comprising a direct current motor, means for impressing said alternating current signal upon a synchronous pilot motor whereby said synchronous motor is continuously operative at the speed of said transmitter, a differential coupling gear between said direct current motor and said synchronous motor having an element responsive to speed differences between said motors, said element cooperating with an electrical time delay circuit to insert resistance into the field circuit of said direct current motor until the speed of said direct current motor rises to a predetermined speed above the speed of said synchronous motor and then to remove resistance from the field circuit of said recording motor until the speed of said recording motor falls to a predetermined speed below the speed of said synchronous motor, said resistance variations being continuous and rapid whereby the average speed of said recording motor is equal to the speed of said synchronous motor.

WILLIAM G. H. F'INCH. 

